As known techniques for supplying power in a wireless manner, there is the technique of supplying power by electromagnetic induction and the technique of supplying power by electric waves. Meanwhile, in recent years, a technique of supplying power by magnetic resonance has been proposed.
In a known wireless power supply system using magnetic resonance, for example, a resonance coil having a particular resonance frequency is disposed in a power transmission device, and a resonance coil having the same resonance frequency as the coil in the power transmission device is disposed in a power reception device. Between the power transmission device and the power reception device, magnetic field coupling is caused, which makes it possible to transfer electromagnetic energy by magnetic resonance. According to the magnetic field coupling, power is transmitted in a wireless manner from the resonance coil of the power transmission device to the resonance coil of the power reception device. With this system, the power transmission efficiency is increased approximately by several tens of percent. This makes it possible to space apart the power transmission device and the power reception device by a relatively long distance. For example, when the sizes of the resonance coils are approximately several tens of cm, the space between the resonance coils may be approximately several tens of cm or more.
A known non-contact power transmission device supplies power from a main unit to a terminal in a non-contact manner, through a first coil of the main unit and a second coil of the terminal. The power transmission device includes an overtemperature prevention device in the main body for automatically stopping the power supply to the terminal when a temperature higher than a predetermined level is detected.
A known non-contact charging device has a charging unit in which a barcode reader equipped with a secondary battery is set in a vertical manner on the top surface of the case of the charging device. On the bottom edge of the main unit case of the barcode reader, a power reception coil is provided. At the bottom of the charging unit of the charging device, a power transmission coil that is electromagnetically coupled with the power reception coil is provided. A temperature sensor for detecting the temperature of the power transmission coil is provided. The charge control circuit controls the charge current so that when the temperature detected by the temperature sensor is less than or equal to a set temperature, the charge current increases, and when the temperature detected by the temperature sensor exceeds the set temperature, the charge current decreases.
In a known non-contact charging device, a temperature detector element is provided at an optimum position that is obtained as follows. The area, the material, and the position of a metallic foreign body placed on the charger and the position of setting the temperature detector element are adjusted, while measuring the temperature rise of the metallic foreign body and the temperature detected by the temperature detector element. According to the measurement results, the optimum position of the temperature detector element is determined. Based on the temperature detected by the temperature detector element, when an abnormal temperature rise of the metallic foreign body is detected, the charging operation is immediately stopped. Specifically, the temperature detector element is provided at a contact surface between the charger and a primary side transmission coil, so that the center of the temperature detector element is located at a position 5 mm away from the center of the primary side transmission coil in the charger. Accordingly, the abnormal temperature rise of the metallic foreign body is accurately detected in a real-time manner, so that safety is enhanced.
A known method of charging a secondary battery involves changing the charge current of the secondary battery according to the temperature rise of the battery. The method of charging the secondary battery involves detecting the charge start temperature which is the battery temperature when the charging starts and the temperature rise gradient by which the temperature of the charged secondary battery rises. According to the detected charge start temperature and temperature rise gradient, the current switching temperature for switching the charge current is changed. Thus, the battery can be rapidly charged with a high current without degrading the battery performance.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2001-258182    Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-153457    Patent Document 3: Japanese Laid-Open Patent Publication No. 2008-172874    Patent Document 4: Japanese Laid-Open Patent Publication No. 2002-10513
The inventor has recognized that, for example, when the power reception resonance coil is not located within a preferable distance from the power transmission resonance coil, and when the power reception resonance coil does not satisfy the resonance conditions, the power transmitted from the power transmission resonance coil may not be sufficiently received by the power reception resonance coil. The inventor has recognized that, for example, when a foreign body satisfying the resonance conditions is located between the power transmission resonance coil and the power reception resonance coil, and when a foreign body that does not satisfy the resonance conditions but that acts as a load is located between the power transmission resonance coil and the power reception resonance coil, the power transmitted from the power transmission resonance coil may not be sufficiently received by the power reception resonance coil. Furthermore, the inventor has recognized that when power transmitted from the power transmission resonance coil is not sufficiently received by the power reception resonance coil, the power transmission resonance coil may produce heat. Thus, the transmitted electromagnetic energy is wasted, and the power transmission coil or the device that is the foreign body may break down.